1. Field of the Invention
This invention relates to drive systems for electrically-powered typewriters and, more particularly, to the selective control of the impression or typing density of such machines.
2. Description of the Prior Art
Adjustment of the driving force delivered to the typing action in electrically-powered typewriters and other business machines is necessary in order to control the impression made by the typebar of the machine on the paper for variation thereof with respect to the number of carbon copies. The accurate control of the energy is a difficult task which has required substantial effort on the part of business machine manufacturers. Prior attempts to control the drive forces transmitted to the typing action have included adjustable intermediate pulleys between the power roll of the machine and the drive motor for the machine, such as U.S. Pat. No. R. 25,024, to vary the speed at which the power roll rotates and thus vary the energy imparted to the typebar by the power roll. This pulley arrangement increases the wear and tear on the drive elements as well as the motor and requires additional costly components. Other approaches have included providing lost motion kinematic connections between the various links of the typing action, such as U.S. Pat. No. 3,578,128, to alter the effective paths of travel of the various members of the typing action and thereby control the initial velocity of the typebar. These lost motion linkage arrangements require substantial amounts of sliding movement between the various linkages in the typing action thus adding to the wear thereof and increasing the cost and complexity of the structure. Another problem with the lost motion linkages is that an individual linkage must be provided for each key so that this arrangement is relatively complex, expensive and sensitive to malfunction.